Calibration of Pulse-Shape Discriminating NaI(Tl) Detectors

This research was sponsored by the National Science Foundation Grant NSF PHY 87-1440

Role of dynamical particle-vibration coupling in reconciliation of the d3/2d_{3/2} puzzle for spherical proton emitters

It has been observed that decay rate for proton emission from $d_{3/2}$ single particle state is systematically quenched compared with the prediction of a one dimensional potential model although the same model successfully accounts for measured decay rates from $s_{1/2}$ and $h_{11/2}$ states. We reconcile this discrepancy by solving coupled-channels equations, taking into account couplings between the proton motion and vibrational excitations of a daughter nucleus. We apply the formalism to proton emitting nuclei $^{160,161}$Re to show that there is a certain range of parameter set of the excitation energy and the dynamical deformation parameter for the quadrupole phonon excitation which reproduces simultaneously the experimental decay rates from the 2$d_{3/2}$, 3$s_{1/2}$ and 1$h_{11/2}$ states in these nuclei.Comment: RevTex, 12 pages, 4 eps figure

Electron-capture branching ratio of Krm81

The branching ratio of the electron-capture decay of Krm81 (13 s) has been measured in order to obtain the rate of the neutrino capture reaction Br81(,re-)81Kr(190.6 keV). This quantity is needed to determine the properties of an Br81 solar neutrino detector. A branching ratio of (3.140.58)×10-5 was obtained, leading to a logft of 5.10-0.07+0.06 for this transition

Fission Hindrance in hot 216Th: Evaporation Residue Measurements

The fusion evaporation-residue cross section for 32S+184W has been measured at beam energies of E_beam = 165, 174, 185, 196, 205, 215, 225, 236, 246,and 257 MeV using the ATLAS Fragment Mass Analyzer. The data are compared with Statistical Model calculations and it is found that a nuclear dissipation strength, which increases with excitation energy, is required to reproduce the excitation function. A comparison with previously published data show that the dissipation strength depends strongly on the shell structure of the nuclear system.Comment: 15 pages 9 figure

Spherical and deformed structures in Pb189

γ-ray spectroscopy of high-spin states of the neutron-deficient nucleus Pb189 has been conducted with the Gd158(Ar36,5n) and Er164(Si29,4n) reactions. With the first of these, detection of evaporation residues and mass gating were used to unambiguously assign a number of prompt γ-ray transitions to Pb189. With the second reaction and a pulsed beam, an isomer with a mean life of 32 μs was found. Although inconclusive, the available evidence favors identification of the isomer with the 332+ state of the ν(i13/2)-3 configuration. The levels observed below the isomer can be identified with states involving three different structures: the neutron (i13/2)-3 multiplet in the spherical well; a prolate-deformed band involving mixed i13/2 neutron orbitals; and a state with the oblate π(2p-2h)0+ν(i13/ 2)-1 configuration. The evidence for structures associated with different shapes is supported by the observation of E0 components in some of the Jπ→Jπ transitions linking them

γ decay from states at low excitation energy in the neutron-deficient isotope, 200Rn, identified by correlated radioactive decay

The low-lying level structure of the neutron-deficient isotope 200Rn has been studied using the 176Hf(28Si,4n) reaction at a beam energy of 142 MeV. Evaporation residues were selected using an in-flight recoil mass separator, the Argonne Fragment Mass Analyzer, and implanted in a double-sided silicon strip detector. Prompt γ rays in 200Rn were observed at the target position using the AYEBALL array of 19 Compton-suppressed germanium detectors, and were identified by the subsequent radioactive decay of associated recoiling ions in the strip detector. Isotopic assignments of the nuclei produced were made on the basis of the mass-to-charge ratio of the recoiling ion and the energy and half-life of its α decay. Previous results concerning transitions in 202Rn were confirmed. The level scheme deduced for 200Rn, compared with those of heavier radon isotopes, is not consistent with the onset of deformation predicted by theoretical calculations. The estimated production cross section for 200Rn in this reaction was 5 μb

Spectroscopy of the proton emitter 109I

Excited states in the proton-unbound nucleus 109I were populated using the 54Fe(58Ni,p2n) reaction at a beam energy of 220 MeV. Gamma rays in 109I were identified using the recoil-decay tagging technique. The analysis of proton-correlated gg coincidence data produced the yrast decay sequence in 109I, which can be tentatively assigned as built on the h11/2 proton state based on systematic trends in the neighboring isotopes. This sequence is completely different from that reported previously. A comparison of the h11/2 band in 109I with those in heavier iodines shows that 109I continues the trend of decreasing quadrupole deformation with decreasing neutron number

Theoretical description of deformed proton emitters: nonadiabatic coupled-channel method

The newly developed nonadiabatic method based on the coupled-channel Schroedinger equation with Gamow states is used to study the phenomenon of proton radioactivity. The new method, adopting the weak coupling regime of the particle-plus-rotor model, allows for the inclusion of excitations in the daughter nucleus. This can lead to rather different predictions for lifetimes and branching ratios as compared to the standard adiabatic approximation corresponding to the strong coupling scheme. Calculations are performed for several experimentally seen, non-spherical nuclei beyond the proton dripline. By comparing theory and experiment, we are able to characterize the angular momentum content of the observed narrow resonance.Comment: 12 pages including 10 figure

Decay properties of the new isotopes 172Hg and 173Hg

The α decays of the two neutron-deficient nuclei 172Hg and 173Hg were observed for the first time using the 78Kr(96Ru,2n) and 80Kr(96Ru,3n) reactions, respectively. The reaction products were dispersed according to their mass-to-charge state ratios in the Argonne Fragment Mass Analyzer and implanted in a double-sided silicon strip detector, where their subsequent decays were studied using spatial and time correlations between implants and decays. A half-life of 250(+350-90) μs and an energy of 7350(12) keV were deduced for the α decay of 172Hg. In 173Hg the half-life was measured to be 0.93(+0.57-0.26) ms and the corresponding energy is 7211(11) keV. In addition, the half-life and energy of the α decay of 174Hg were measured more precisely. The reduced widths deduced for these Hg isotopes indicate that the observed decays correspond to unhindered Δl = 0 transitions. The α-decay Q values are compared with the values calculated using mass tables by Möller and Nix, and by Liran and Zeldes. The latter mass tables show better agreement with the data

Proton and α radioactivity of 185Bi

Proton and α emission from 185Bi have been confirmed and measured with improved statistics. The 185Bi nuclei were produced via the 95Mo(92Mo,pn) reaction at a bombarding energy of 420 MeV. The proton decay energy from the 1/2+ intruder state in 185Bi to the 184Pb ground state was measured to be 1.598(16) MeV with a proton branching ratio bp, = 0.85(6). An α decay branch from the same state was measured, bα = 0.15(6), with an energy of 8.08(3) MeV. The state has a half-life of 50(8) μs. In addition, the α branching ratio of the ground state of 184Pb was determined for the first time to be bα = 0.23(14)